Textbook Question
Propose mechanisms for the following base-catalyzed condensations, with dehydration.
(a) 2,2-dimethylpropanal with acetaldehyde
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Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds
Wade9th EditionOrganic ChemistryISBN: 9780135213728
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Table of contents
- Ch.1 - Structure and Bonding107
- Ch. 2 - Acids and Bases; Functional Groups115
- Ch.3 - Structure and Stereochemistry of Alkanes100
- Ch.4 - The Study of Chemical Reactions99
- Ch.5 - Stereochemistry93
- Ch.6 - Alkyl Halides; Nucleophilic Substitution118
- Ch. 7 - Structure and Synthesis of Alkenes; Elimination158
- Ch.8 - Reactions of Alkenes171
- Ch.9 - Alkynes91
- Ch.10 - Structure and Synthesis of Alcohols143
- Ch.11 - Reactions of Alcohols104
- Ch. 12 - Infrared Spectroscopy and Mass Spectrometry22
- Ch. 13 - Nuclear Magnetic Resonance Spectroscopy45
- Ch. 14 - Ethers, Epoxides, and Thioethers72
- Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy89
- Ch. 16 - Aromatic Compounds74
- Ch. 17 - Reactions of Aromatic Compounds126
- Ch. 18 - Ketones and Aldehydes193
- Ch. 19 - Amines129
- Ch. 20 - Carboxylic Acids77
- Ch. 21 - Carboxylic Acid Derivatives141
- Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds175
- Ch. 23 - Carbohydrates and Nucleic Acids93
- Ch. 24 - Amino Acids, Peptides, and Proteins54
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Wade 9th EditionCh. 22 - Condensations and Alpha Substitutions of Carbonyl CompoundsProblem 25b
Wade 9th EditionCh. 22 - Condensations and Alpha Substitutions of Carbonyl CompoundsProblem 25bChapter 22, Problem 25b
Propose mechanisms for the following base-catalyzed condensations, with dehydration.
(b) benzaldehyde with propionaldehyde
Verified step by step guidance1
Identify the reaction type: This is a base-catalyzed aldol condensation reaction, which involves the formation of a β-hydroxy aldehyde or ketone followed by dehydration to form an α,β-unsaturated carbonyl compound.
Deprotonation of the α-hydrogen: The base (e.g., hydroxide ion, OH⁻) abstracts an α-hydrogen from propionaldehyde, generating an enolate ion. The enolate ion is stabilized by resonance, with the negative charge delocalized between the oxygen and the α-carbon.
Nucleophilic attack: The enolate ion acts as a nucleophile and attacks the carbonyl carbon of benzaldehyde. This forms a new carbon-carbon bond, resulting in an alkoxide intermediate.
Protonation of the alkoxide: The alkoxide intermediate is protonated by water (or another proton source), forming a β-hydroxy aldehyde. This is the aldol addition product.
Dehydration step: Under basic conditions, the β-hydroxy aldehyde undergoes elimination of water. The base abstracts a proton from the β-carbon, and the hydroxide group leaves, forming a double bond between the α and β carbons. The final product is an α,β-unsaturated aldehyde.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Base-Catalyzed Condensation
Base-catalyzed condensation reactions involve the formation of a new bond between two carbonyl compounds, facilitated by a base. The base deprotonates an alpha hydrogen, generating an enolate ion that can attack the carbonyl carbon of another aldehyde or ketone. This process typically leads to the formation of a β-hydroxy carbonyl compound, which can subsequently undergo dehydration to yield an α,β-unsaturated carbonyl compound.
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Base Catalyzed
Mechanism of Aldol Condensation
The aldol condensation mechanism consists of two main steps: the formation of an aldol (β-hydroxy carbonyl) and its subsequent dehydration. Initially, the enolate ion formed from one carbonyl compound attacks the carbonyl carbon of another, resulting in a β-hydroxy carbonyl. The dehydration step involves the elimination of a water molecule, leading to the formation of a double bond and yielding an α,β-unsaturated carbonyl compound, which is often more stable and reactive.
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Dehydration in Organic Reactions
Dehydration in organic chemistry refers to the removal of a water molecule from a compound, often occurring in condensation reactions. In the context of aldol condensation, dehydration typically follows the formation of a β-hydroxy carbonyl compound, resulting in the formation of a double bond. This step is crucial as it enhances the stability of the product and increases its reactivity, making it a key feature in many organic synthesis pathways.
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Related Practice
Textbook Question
Predict the products of aldol condensation, followed by dehydration, of the following ketones and aldehydes.
(b) acetophenone
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Textbook Question
Predict the products of aldol condensation, followed by dehydration, of the following ketones and aldehydes.
(c) cyclohexanone
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Textbook Question
Predict the major products of the following base-catalyzed aldol condensations with dehydration.
(a) benzophenone (PhCOPh) + propionaldehyde
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Textbook Question
Predict the major products of the following base-catalyzed aldol condensations with dehydration.
(b) 2,2-dimethylpropanal + acetophenone
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Textbook Question
Show how octane-2,7-dione might cyclize to a cycloheptenone. Explain why ring closure to the cycloheptenone is not favored.
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